Cleanup internals.configurationForViewport and prepare it for future changes

The method is now prepared for the upcoming viewport meta -> @viewport
tranlation, by correctly setting the initial viewport size.

Review URL: https://chromiumcodereview.appspot.com/19772003

git-svn-id: svn://svn.chromium.org/blink/trunk@154491 bbb929c8-8fbe-4397-9dbb-9b2b20218538